PSAs are a distinct category of adhesives which, in dry (solvent-free) form, are aggressively and permanently tacky at room temperature and firmly adhere to a variety of dissimilar substrates upon mere contact, without need of more than finger or hand pressure. PSAs do not require activation by water, heat, or solvents; and have sufficient cohesive strength to be handled with the fingers. The primary mode of bonding for a PSA is not chemical or mechanical but, rather, a polar attraction to the substrate, and always requires initial pressure to achieve sufficient wet-out onto the surface to provide adequate adhesion.
Both rubber-based and acrylic-based PSAs are known. In 1966, C. Dalquist identified a one-second creep compliance greater than 1×10−6 cm2/dyne as the efficient contact criterion for a good PSA. A more recent discussion of PSAs in the Handbook of Pressure Sensitive Adhesive Technology (2d Edition), D. Satas, ed. (1989), (hereafter, “Handbook”), pages 172-176, incorporated by reference herein, identifies glass transition temperature (Tg) and modulus (G′) at the application (use) temperature as the most important requirements for PSA performance. Both properties are a function of the identities and amounts of monomers that comprise the PSA polymer(s). Thus, poly(acrylic acid) is not a PSA, but a copolymer of acrylic acid and a high mole % of 2-ethylhexyl acrylate is.
The typical values of G′ and Tg for label and tape PSAs are described in the Handbook. For a tape, G′ at room temperature≈5×105 to 2×106 dyne/cm2, and Tg≈−15° C. to 10° C.; while labels have a lower value of G′ at room temperature, i.e., about 2×105 to 8×105 dyne/cm2. Tg requirements for cold temperature, permanent, and removable applications are different, as is known in the art. Thus, cold temperature label PSAs generally require a Tg of from about −30° C. to −10° C.
High performance PSAs are normally characterized by the ability to withstand creep or shear deformation at high loadings and/or high temperatures, while exhibiting adequate tack and peel adhesion properties. A high molecular weight provides the necessary cohesive strength and resistance to shear deformation, while a low modulus allows the polymer to conform to a substrate surface upon contact.
High molecular weight, or the physical effect of a high molecular weight, can be obtained by primary polymerization of monomers to form a backbone of long chain length, and/or by creating a high degree of inter chain hydrogen bonding, ionic association, or crosslinking between polymer chains. For solvent-based adhesives, it is preferred to crosslink after polymerization (so-called “post-polymerization crosslinking), which avoids processing difficulties such as coating a highly viscous polymer network. Post-crosslinking is also commonly used for water-based PSAs to enhance cohesive strength. Post-curing is also sometimes used with hot melt PSAs, although radiation curing is more commonly employed with such systems, to avoid thermal cure during the coating process.
Thermal crosslinking and photoinitiated crosslinking are well-known approaches to introducing crosslinks between polymer chains. In most photoinitiated (photocuring) systems, a post-added photoinitiator is employed and reacts with an acrylate, methacrylate, allyl, epoxy, or other functional group on the polymer or oligomer to be cured. Ultraviolet (UV) radiation causes curing. In such systems, a photoinitiator residue remains, and can have a deleterious effect. For example, in medical applications, such residues can cause skin irritation. In electronics applications, the residues can introduce undesirable contamination to the coated device. In addition, the curing process used in post-added photoinitiator systems is usually oxygen-sensitive. There is a need for high performance PSAs that have a good balance of tack, peel and shear strength, and that do not suffer from the drawbacks of photoinitiator residues and incomplete curing due to the presence of oxygen.
Another area where improved polymeric compositions are needed is protective coatings. These coatings include marine coatings, coatings for automotive components, scratch-resistant and dust-resistant coatings, top coats for printed or imprintable materials, and other coatings that provide a substrate with a protective barrier. For some applications, processing constraints limit the types of coatings that will work. For example, many electronic components are damaged by high temperatures and are thus unsuitable for protection with thermally cured coatings, unlike automotive finish applications. UV-curable coatings are the preferred choice for electronic components. However, electronic components are also sensitive to contaminants, particularly if the contaminants can cause a build-up in static charge or a change in the magnetic properties of the component. A need exists for improved protective coatings for electronic components.
Polymeric protective coatings having a low surface energy and chemical inertness are also desired in a variety of applications. Low surface energy materials are important for applications requiring reduced wetting and adhesion to other compounds; for instance, they can be used as coatings for supressing marine bioadhesion, for membranes with reduced biofouling, and as protective implants. Typically, such coatings have a surface energy of 16-25 dyne/cm. Fluropolymeric coatings are representative. An inherent drawback of such low surface energy, fluropolymeric coatings is poor adhesion to some substrates, such as ABS. This can lead to premature deprotection of the surface. On the other hand, acrylic polymers or resins have been widely used as coatings for many applications due to their good adhesion properties, UV stability, and optical clarity, etc. However, the higher surface energy (>33 dyne/cm) of acrylic polymers excludes them from many of the above-mentioned applications. Although in recent years, efforts have been made in the synthesis of fluorinated acrylic coatings through copolymerization, surface grafting, and polymer blend techniques, the use of acrylic polymers with fluropolymer blocks for coatings has not received much attention. A need exists for improved acrylic-fluropolymeric hybrids useful in a variety of coating applications.